ClojureScript × Type Inference

1. ClojureScript Type Inference
3. Type Inference a conclusion reached on the basis of evidence and reasoning
4. Type Inference a conclusion reached on the basis of evidence and reasoning 2019 is it a number? "2019" is it a number? can I add them?
5. Why in ClojureScript? Generate optimal JavaScript Catch stupid bugs at compile time Improve externs inference
6. (if (yes) (no)) if (cljs.core.truth_( )) { yes(); } else { no(); }
7. (if ^boolean (yes) (no)) if ( ) { yes(); } else { no(); } “type hint”
8. Types 'string 'number 'boolean 'seq 'array 'object 'any 'clj-nil nil
9. Types (defn f [x] (if x 1 "2")) #{number string}
10. Compiler phases Read Analyze Emit
11. Compiler phases Read Analyze Emit "(let [x 1] ...)"
12. Compiler phases Read Analyze Emit {:op :let :body ... :bindings [{:name 'x :init {:op :const}} ...]}
13. Compiler phases Read Analyze Emit var x = 1;
14. Analyzer (defn weirdo [a b] (str a b)) (+ 1 (weirdo 2 3))
15. Analyzer 1 + weirdo(2, 3) WARNING: cljs.core/+, all arguments must be numbers, got [number string] instead
16. Analyzer WARNING: cljs.core/+, all arguments must be numbers, got [number string] instead (defn weirdo [a b] (str a b)) (+ 1 (weirdo 2 3))
17. Return Type Inference (defn ^string str [a b] ...) (defn weirdo [a b] (str a b)) (+ 1 (weirdo 2 3))
18. String Coercion Elision (str a b) [str(a), str(b)].join("");
19. String Coercion Elision (str ^string a ^string b) [a, b].join("");
20. Predicate-Induced Inference (if (string? x) (str x "y") :else)
21. Specializations (def x "string") (def y #js [1 2 3]) (count x) (count y)
22. Specializations (def x "string") (def y #js [1 2 3]) x.length y.length
23. Externs Inference (defn http-get [url] (js/fetch url)) (-> (apply http-get "example.com") (.then on-ok) (.catch on-error))
24. Externs Inference (defn http-get [url] (js/fetch url)) (-> ^js/Promise (apply http-get "example.com") (.then on-ok) (.catch on-error))
25. Experiments
26. Dead Code Elimination (if x :then :else) 'clj-nil :else
27. Dead Code Elimination (if x :then :else) #{number string} :then
28. Specializations (def x "string") (def y #js [1 2 3]) (first x) (first y) x[0] y[0]
29. Specializations (def x "string") (def y #js [1 2 3]) (second x) (second y) x[1] y[1]
30. Specializations (def x "string") (def y #js [1 2 3]) (empty? x) (empty? y) x.length === 0 y.length === 0
31. Higher-order inference (identity 1) 'any (identity 1) 'number
32. Higher-order inference (apply str xs) 'any (apply str xs) 'string
33. Higher-order inference (defn f [] ([] 1) ([x] (str x "y"))) (apply f xs) #{number string} (apply f "x" xs) 'string
34. Higher-order inference (map inc xs) ['seq 'number] (first xs) 'number
35. Type Inference + clojure.spec (defui component [x] [:h1 x]) attributes map or child element?
36. Type Inference + clojure.spec (s/fdef component :args (s/cat :x map? :y string?)) (defui component [x y] [:h1 x (str "Hello, " y)])
37. Summary Compiler gets smarter Your code gets faster without you doing anything You don't have to know all of this But it might be useful to optimize hot path